Unit Manufacturing Processes Issues and Opportunities in Research (1995) / Chapter Skim
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13 Process Precision and Metrology
13 Process Precision and Metrology
Pages 153-172
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From page 153... ...
This chapter deals with process precision, a crucial but often overlooked component of quality technology. Spatial precision (i.e., issues of form and fit)
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From page 154... ...
Variations in part geometry, as graphically illustrated in Figure 13-1, could result from inherently imprecise processes, or from variations in process control. Control variations could be due to a lack of knowledge concerning the process variables, inadequate means of process control, indifference to process control, etc.
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From page 155... ...
Thus three key producibility themes emerge: design to accommodate variability arising from the control process design, design to ensure that the realized process variations do not exceed the design tolerance, and design to minimize the dispersion of variations within the allowed ranges of variability through careful control of manufacturing and assembly processes. The nominal design of products, subassemblies, and parts is driven mainly by functionalism that is, by what the items must do.
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From page 156... ...
Some of the current topics in process precision and metrology are discussed below. They include issues in dimensional scale and precision in manufacturing, dimensional tolerances and metrology, process planning, and process modeling.
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From page 157... ...
Figure 13-2 distinguishes "precision" and "ultraprecision" machining from "normal" machining in terms of dimensional scale and tolerance. Precise and ultraprecise manufacturing and measurement processes are quite specialized and limited in applicability, but the volume, value, and technical importance of the products requiring processing in these regimes are growing.
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From page 158... ...
158 Unit Manufacturing Processes: Issues and Opportunities in Research Table 13-1 Dimensional Scale and Precision for a Range of Manufactured Items (Swyt, 1992) Part or Geometric Process Dimension Tolerance Ratio Sub- Attribute D T T/D assembly Auto door Panel size/ Die 1 m 1 mm 10-3 assembly position stamping Auto Diameter/ Machining 100 mm 7-8 mm 104 engine cylindricity piston Magnetic Cut-face read/ write heads .
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From page 159... ...
They are likely to be replaced gradually with faster noncontacting technologies based on wave phenomena.
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From page 160... ...
When CMMs arrived, however, inspectors found that CMM results did not always agree with traditional inspection results. This is called "methods divergence," and its recognition triggered increasingly strident warnings in the 1980s about a "metrology crisis." A second problem, "specification ambiguity," was exposed when CMM and computer-aided design programmers found that some of the prose and graphic definitions in Y14.5 are ambiguous.
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From page 161... ...
It is worth noting that there is a movement in Europe advocating the adoption of vectorial tolerancing as a replacement for, or at least a co-equal alternative to, the International Standards Organization brand of geometric tolerancing. Vectorial tolerancing was formulated by Adolph Wirtz of Switzerland (Wirtz, 1991~; Figure 13-3 conveys some of its essential elements.
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From page 163... ...
TO -~~3 o $ to x \ - -a r O ~ ~ J T 1 1 o _ for \ 163 ~3 ~ < hi' o ~ ~ $° o o \1 ~ ~ 8 Cot CC x ao so ~4 ._
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From page 164... ...
design would have been a major activity. Experienced machinists can easily construct plans such as that shown in Figure 13-5, because they possess a wealth of experiential data arid subtle reasoning powers.
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From page 165... ...
The entire area of process-induced form modeling requires a significant increase in research, because there are few process models that provide explicit representations of the forms that can be produced, and even fewer that provide the precision data useful for manufacturing planning and tooling design. Machining has been studied longer (for more than 100 years)
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From page 166... ...
For example, mathematical and computational tools offered by solid modeling enable the nominal form-modifying effects of numerical control machining to be modeled mathematically and simulated computationally with increasing fidelity (Menon and Robinson, 1993; Menon and Voelcker, 1993~. Research on improved precision models for machining is more scattered, but several references provide useful introductory discussions and references (See Chang et al., 1991; Wood, 1993, for a different approach based on fractal theory)
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From page 167... ...
To pursue this work, precision engineers have had to probe deeply into the sources of process alla machine variability, and thus process engineers should participate, or even take the lead, in building process-precision models (Slocum, 19921. Process planning, as a distinct activity and as an area for formal study, has a much shorter history.
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From page 168... ...
It is probably safe to predict that automatic process planning for manufacturing will not progress very far until better process and process-precision models are available.
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From page 169... ...
In design, the primary mechanisms are dimensional, and surface tolerances assigned to ensure interchangeable assembly while preserving function. Dimensional tolerances are governed by national and international standards and are required to be manufacturing-process independent.
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From page 170... ...
· The efforts now under way to mathematize and generalize Y14.5, the American national tolerancing standard, and to define systematic measurement procedures based on a rigorous tolerance standard, should be encouraged. The harmonization of the American National Standards Institute and the International Standards Organization standards should also be encouraged.
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From page 171... ...
American National Standards Institute Standard Y14.5M 1982. New York: American Society of Mechanical Engineers.
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From page 172... ...
~ 993. Fractal-based tolerancing: Theory, dynamic process modeling, test bed development and experiments.
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